The present invention relates to a metallization forming method, more particularly, to a method of forming metallization on printed circuit boards.
Conventionally, the pattern on the metallized part (conductor part) of printed circuit boards (electronic circuit boards) has been formed by two major processes, the subtractive and the additive (semi-additive).
In the subtractive process, the unwanted part of a metal layer on a substrate is removed but the necessary part is left intact to form a metallization pattern (conductor part); in contrast, the additive process involves forming a metallization pattern on a substrate.
In either process, the photolithographic technology has heretofore been employed. In the additive process, metal is reductively precipitated on a surface of a substrate by, for example, electroless plating via a mask so as to form a metallization pattern and the photolithographic technology is used to form the pattern of the mask.
In the subtractive process, a photoresist (a light-sensitive resin) such as a dry film resist or a liquid resist applied to the surface of a copper-clad laminate having an insulating layer and a copper foil as a conductor metal is irradiated with an electromagnetic radiation such as UV light or excimer laser via a photomask having a circuit or other pattern formed thereon and, then, the mask pattern is exposed (transferred) and developed to form a resist pattern.
After thus forming the resist pattern by the photolithographic technology, the copper foil not covered with the photoresist is removed by etching to form a desired metallization pattern.
The above-described method of forming the desired metallization pattern using the resist pattern formed by the photolithographic technology takes time to prepare the photomask and in addition to the etching step for removing the unwanted part of the conductor metal, the step of exposing and developing the resist is required to form the resist pattern. Hence, it takes time and cost to form the metallization pattern.
As another approach, it has recently been proposed to form the pattern of the metallized part (conductor part) of a printed wiring board (electronic circuit board) by using a conductive fine particle dispersed ink system. In the conductive fine particle dispersed ink drawing system, the ink-jet printing process is used to form a desired metallization pattern by directly patterning a conductive, fine particulate material on the substrate in accordance with the metallization pattern.
Since this conductive fine particle dispersed ink drawing system has no need to prepare a mask, it involves fewer steps than the method of forming a desired metallization pattern by applying the photolithographic technology to form a resist pattern.
However, to ensure that the fine particulate material manifests its conductivity, baking must be performed under elevated temperatures for a prolonged period of time; this not only limits the type of substrates (substrate materials) on which a metallization pattern can be formed but it also increases the running cost and the size of the apparatus for performing the baking step.
As alternatives to these methods, it has most recently been proposed that a desired metallization pattern be formed by performing exposure and development processing on the light-sensitive material.
For example, JP 2006-352073 A discloses a method in which a light-sensitive material having a silver salt containing layer on the entire surface of a substrate (support) is exposed using a laser beam based scan exposure system and a photomask and the material is then developed to form a conductive pattern on the substrate, followed by plating the conductive pattern so that it becomes conductive to thereby form a metallization pattern.
JP 2007-129205 A discloses a method in which a light-sensitive material having a silver salt emulsion containing emulsion layer formed on the entire surface of a substrate (support) is exposed using a laser beam based scan exposure system and a photomask and the material is then developed to form metallic silver on the substrate, followed by smoothing the metallic silver to form a metallization pattern.
The metallization pattern forming methods that are disclosed in the two patent documents are described below by referring to FIGS. 10A through 10F, which illustrate the conventional method of forming a metallization pattern.
First, a subbing coat 202 is formed on the entire surface of a substrate S and an emulsion having a silver halide to gelatin ratio of 2:1 by volume is applied to the entire surface of the substrate S to form an emulsion layer 204 (see FIG. 10A).
Then, the emulsion layer 204 is exposed by an analog exposing method via a mask 208 to print the pattern of the mask 208 on the emulsion layer 204 (see FIG. 10B). Instead of the mask 208, a laser exposure direct drawing method may be employed to expose the emulsion layer 204 so that the pattern of the mask 208 is printed on the emulsion layer 204.
The emulsion layer 204 having the pattern of the mask 208 printed on it is developed to generate conductive silver 212 (see FIG. 10C). Since the silver halide is also present in the unexposed areas 214 of the emulsion layer 204, fog, the adhesion of the silver in the developer and other troubles may occur on account of the pressure applied to the emulsion layer 204 or the static electricity generated during the transport of the substrate S, occasionally causing metallic silver (black pepper) 216 to appear in unexposed areas 214.
Accordingly, in order to remove the silver halide from the unexposed areas 214 and stabilize them, fixing, acid washing, the addition of a pressure fog preventing agent, an antistatic agent or the like, and other treatments are performed, followed by washing with water and drying (see FIG. 10D).
Subsequently, in order to increase its conductivity, the conductive silver (metallization pattern) 212 is subjected to smoothing (compacting) such as calendering (see FIG. 10E).
To render the conductive silver 212 even more conductive, plating is performed to form conductive metal parts 218 on top of the conductive silver 212 (see FIG. 10F).
This is the recent method in which a light-sensitive material is applied to the entire surface of a substrate, a mask pattern is printed, and exposure and development processing is performed to form a desired metallization pattern.